The Fishy Side Effects of Pharmacological Waste In Our Waterways

by Dr Catriona Nguyen-Robertson MRSV

This piece appears in the May 2023 edition of Science Victoria magazine. All issues can be read online for free at

When we think of pollution in streams and rivers, we tend to think first of rubbish in water; the litter traps along the Yarra River in Melbourne’s CBD that are often overflowing, or the empty plastic bottles along the Moonee Ponds Creek.

But water pollution takes many forms, from physical trash to invisible chemicals that also accumulate in our waterways. Emerging contaminants like pharmacological waste and microplastics are not filtered out by current wastewater treatment plants. These chemicals instead flow into water ecosystems where they can have devastating ecological impacts.

We are literally medicating our waterways. Pharmacological waste is now ubiquitous in the environment and many drugs have long half-lives, persisting even in the most remote places on the planet, including Antarctica.

Pharmaceutical drugs enter our water supply when people release traces in their urine (or flat-out flush unused medication down the sink or toilet). Surprisingly, 50-60% of the active ingredients of some pharmaceuticals, such as oestrogen in the conception pill, pass through our bodies and are flushed out in urine. These active ingredients subsequently pollute waterways, virtually unchanged even by wastewater treatment, and now many wild fish are swimming in an active drug cocktail that contaminates rivers and streams.

These compounds – including antidepressants, painkillers and blood-pressure medicine – are working their way through the food web. In 2018, freshwater ecologist Dr Erinn Richmond and her colleagues at the Water Studies Centre, Monash University detected 69 out of 98 pharmaceutical compounds tested for in aquatic insects living in Victorian creeks.1 The most commonly detected pharmaceuticals in the tissues of these insects were memantine (a treatment for Parkinson’s); codeine; fluconazole (an antifungal medicine); metoprolol (a treatment for high-blood pressure and angina); and mianserin (an antidepressant). Alarmingly, these compounds were also detected in spiders that feed on aquatic insects, highlighting that predators further up the food chain are potentially also exposed to high levels of drugs.

A large issue with pharmaceuticals is that the receptors on our cells that drugs are designed to target tend to be evolutionarily conserved among animals. Medicines that are developed for humans can therefore have similar effects on other non-target species. However, only a fraction of the dose used for humans is needed to affect smaller animals – and the effects can be quite drastic.

Psychoactive drug and antidepressant pollutants have been found in water habitats all around the world, including here in Australia.2 They have been detected in drinking water, surface water, ground water, seawater, and other water bodies. The purpose of these drugs is to alter behaviour and mood in people – and they are starting to do that in wildlife too. Professor Bob Wong’s team at Monash University found that exposure to the antidepressant fluoxetine (Prozac®), disturbed a freshwater fish’s foraging behaviour or ability to escape from predators.3 Additionally, an anti-anxiety drug, oxazepam, is present in the Fyris River in Sweden at concentrations that can affect fish behaviour, making fish less social, more active, faster feeders, and more bold.4 Fish with drug-enhanced appetites could more quickly deplete their food resources, and bold, active fish that do not congregate with others can also make themselves easier targets for predators. These drugs that accumulate in waterways thus negatively impact the ability of fish to survive.

Another drug that ends up in waterways is the contraceptive pill, containing progesterone and oestrogen. These so-called hormone-disruptors have unintended consequences in aquatic animals as it disturbs their normal, delicate hormone balance. Professor Wong’s research into this area began 15 years ago when he observed declining biodiversity in the streams of Mexico as female sawtail fish struggled to seek males of their own species and were instead mating with the wrong species.5

In addition to behavioural changes, studies have seen abnormalities in the genitalia of both terrestrial and aquatic life due to exposure to hormone-disruptors like the contraceptive pill. Male fish exposed to hormone-disruptors can become feminised, in which female egg cells grow in their testes. Some fish species, such as clown fish, are naturally hermaphroditic and can change sex to increase their chances of reproducing. However, when female eggs are present in male fish that are not hermaphroditic, it greatly impedes their reproductive success. Over the past decade, feminised male fish have been discovered in 37 species in lakes and rivers throughout North America, Europe, and other parts of the world.6 Furthermore, an astonishing 60 to 100 percent of all the male smallmouth bass examined in 19 US National Wildlife Refuges have become feminised.7 A similar feminisation phenomenon has also been seen in alligators, turtles and frogs. Pharmacological contaminants in water hence negatively influences the fertility and reproductive capacity of animals, and ultimately, biodiversity in aquatic ecosystems worldwide.

There is no easy solution to prevent drugs from entering our waterways. We cannot simply get rid of or ban all these products given that many people take them out of necessity. We can, however, be more considerate of their whole lifecycle. Could we get pharmaceutical companies to research the impact of their products once we have consumed them? Can we get consumers to be more mindful, especially when disposing unused tablets? And can we further improve their removal in wastewater treatment plants? It is encouraging to see some governments and environmental agencies begin to assess different methods of removing hormones during sewage treatment. For example, a wastewater treatment facility in Canada has made small upgrades to reduce oestrogen levels, leading to a drop in incidences of feminised rainbow darter fish found downstream.8

When we take pharmaceutical drugs, they are not always entirely used within our bodies, and they therefore have these unintended consequences. The drugs we take have a targeted physiological response on humans – that is why we take them. However, we do need to be more considerate about how the pollution from the waste we wee impacts fauna in rivers and streams.


  1. Richmond, E.K. et al. (2018). ‘A diverse suite of pharmaceuticals contaminates stream and riparian food webs’. Nature Communications. 9.
  2. Mole, R.A. and Brooks, B.W. (2019). ‘Global scanning of selective serotonin reuptake inhibitors: occurrence, wastewater treatment and hazards in aquatic systems.’ Environmental Pollution. 250, pp 1019-1031.
  3. Martin, J.M. et al. (2019). ‘Field-realistic antidepressant exposure disrupts group foraging dynamics in mosquitofish’. The Royal Society Publishing. 15(11).
  4. Brodin, T. et al. (2013). ‘Dilute Concentrations of a Psychiatric Drug Alter Behavior of Fish from Natural Populations’. Science. 339(6121), pp 814-815.
  5. Fisher, H.S, et al. (2006), ‘Alteration of the chemical environment disrupts communication in a freshwater fish’. Royal Society Publishing. 273(1591).
  6. Bahamonde, P.A. et al. (2013). ‘Intersex in teleost fish: Are we distinguishing endocrine disruption from natural phenomena?’ General and Comparative Endocrinology. 192, pp 25-35.
  7. Iwanowicz, L.R. et al. (2016). ‘Evidence of estrogenic endocrine disruption in smallmouth and largemouth bass inhabiting Northeast U.S. national wildlife refuge waters: A reconnaissance study’. Ecotoxicology and Environmental Safety. 124, pp 50-59.
  8. Hicks, K. et al. (2016). ‘Reduction of Intersex in a Wild Fish Population in Response to Major Municipal Wastewater Treatment Plant Upgrades’. Environmental Science and Technology. 51(3).